This R21 proposal, which is responsive to PA-03-065 "Calcium oxalate Stone Diseases", builds on our recent demonstration of the presence of glyoxylate reductase/hydroxypyruvate reductase in mitochondria. Glyoxylate Reductase/Hydroxypyruvate Reductase (GRHPR) is a mutifunctional oxidoreductase that catalyzes conversion of glyoxylate to glycolate (GR), conversion of hydroxypyruvate to D-glycerate (HPR), and conversion of D-glycerate to hydroxypyruvate (DGDH). Our preliminary data demonstrate that GRHPR enzymatic activity is present in most, if not all, human tissues raising questions about the role of GRHPR in non-hepatic sites. We have also demonstrated that GRHPR is located both in the cytoplasm and in mitochondria. We hypothesize that mitochondrial targeted GRHPR is important for detoxification of glyoxylate produced by the mitochondrial hydroxyproline catabolism that occurs in hepatic and renal cells, or by other unidentified mechanisms in non-hepatic cells. Further analysis of the molecular biology of GRHPR demonstrates the presence of a putative mitochondrial localization sequence. We hypothesize that a subset of PH2 patients with mutations prior to this localization sequence will retain mitochondrial GRHPR expression, but lack cytoplasmic GRHPR. These patients may have a less severe phenotype than patients that lack both cytoplasmic and mitochondrial GRHPR. Further, we hypothesize that there is a subset of hereditary hyperoxaluria patients, currently referred to as unclassified hyperoxaluria patients (UncPH) that possess a mutation in the mitochondrial targeting sequence of GRHPR but produce functional cytoplasmic GRHPR. This proposal will define the mitochondrial targeting localization sequence in GRHPR, determine the role of alternative splice variants in regulating GRHPR intracellular localization, and define the role of GRHPR mistargeting in primary hyperoxaluria. The studies in this proposal could uncover the molecular basis of UncPH, develop new screening tools for PH, and elucidate basic mechanisms that dictate the intracellular targeting of GRHPR. [unreadable] [unreadable] [unreadable]